The invention relates to an electrode for electrolytic processes, in particular to an anode suitable for oxygen evolution in an industrial electrolysis process. Anodes for oxygen evolution are widely used in different electrolytic applications, many of which relating to the field of cathodic electrodeposition of metals (electrometallurgy), working in a wide range of applied current density, from very low (a few hundred A/m2, such as in metal electrowinning processes) to extremely high (as in some galvanic electroplating applications, which can operate in excess of 10 kA/m2, with reference to the anodic surface); another field of application of anodes for oxygen evolution is cathodic protection by impressed current. In the field of electrometallurgy, with particular reference to metal electrowinning, lead-based anodes are traditionally used, still valid for certain applications although presenting a rather high oxygen evolution overpotential and also entailing well-known risks for the environment and human health. More recently, electrodes for anodic oxygen evolution obtained from substrates of valve metals, for example titanium and its alloys, coated with catalyst compositions based on metals or oxides thereof were introduced in the market, especially for high current density applications, which benefit the most of the energy savings associated with a decreased oxygen evolution potential. A typical composition suitable to catalyse the anodic oxygen evolution reaction consists for instance of a mixture of oxides of iridium and tantalum, wherein iridium is the catalytically active species and tantalum facilitates the formation of a compact coating, capable of protecting the valve metal substrate from corrosion, particularly for operation in aggressive electrolytes. Another very effective formulation for catalysing the anodic oxygen evolution reaction consists of a mixture of oxides of iridium and tin, with small quantities of doping elements such as bismuth, antimony, tantalum or niobium, useful to make the tin oxide phase more conductive.
An electrode with the above composition is capable of satisfying the needs of many industrial applications, both at low and at high current density, with sufficiently reduced operating voltages and reasonable durations. The economy of certain manufacturing processes especially in the domain of metallurgy (such as copper or tin electrowinning) nevertheless requires electrodes of even higher duration than the above compositions. To achieve this goal, protective intermediate layers are known based on valve metal oxides, for example mixtures of tantalum and titanium oxides, capable of further preventing the corrosion of the valve metal substrate. The intermediate layers thus formulated are nevertheless characterised by a rather low electric conductivity and can only be used at a very reduced thickness, not exceeding 0.5 μm, so that the resulting increase in the operating voltage is contained within acceptable limits. In other words, a compromise must be found between a suitable operational lifetime, favoured by a higher thickness, and a reduced overpotential, favoured by a lower one.
Another problem observed with the above described catalytic formulations is the tendency of iridium-containing catalytic coatings to leach a sensible amount of iridium into the electrolyte during the start-up phase and the first hours of operation. This seems to suggests that a fraction of the iridium oxide of the coating, although electrochemically active, is present in a phase less resistant to corrosion by the electrolyte. This phenomenon, which to a certain extent takes place also with other noble metal catalysts such as ruthenium, can be mitigated by overlaying porous protective layers to the catalytic coating, for example based on tantalum or tin oxide. Such external protective layers, however, have a limited effectiveness and cause an increase in the operating voltage of the electrode.
It has thus been evidenced the need to provide anodes for oxygen evolution characterised by an enhanced operational duration and by a reduced release of noble metals in the first hours of operation, while presenting a very high catalytic activity towards the oxygen evolution reaction.